Modern military communication systems require antennas that are small, broadband and have good gain. It is difficult to make electrically small antennas (i.e. less than ¼-wavelength in maximum linear dimension) broadband without seriously impacting gain performance.
One widely used method of making an electrically small antenna broadband is to add a resistive taper along the length of the antenna. This taper is usually determined empirically. More recently, people have applied genetic algorithms (GA) to arrive at an optimal taper.
A major disadvantage of the resistive taper is the amount of resistance that needs to be added to an electrically small antenna in order to achieve a good VSWR (i.e. <3:1) which severely reduces the gain of the antenna due to ohmic losses. Additionally, the taper affects gain at all frequencies since it is in series with the element.
The problem with all broadband antennas is to make sure the VSWR is uniformly low, less than 3:1, over the entire band. What often occurs is that there are large VSWR spikes which can exceed 10:1. Since the VSWR spikes induce mismatch loss, this significantly diminishes the ability to pump in energy to the antenna at various points in the broad frequency band that the antenna is to operate over.
What is desired is to have a broadband antenna which does not have VSWR spikes so that it has a uniform operation across the band.
Broadband meander line loaded antennas are described in U.S. Pat. Nos. 5,790,090; 6,313,716; 6,323,814; 6,373,440; 6,373,446; 6,480,158; 6,492,953; and 6,404,391, incorporated herein by reference and assigned to the assignee hereof. U.S. Pat. No. 6,590,543 describes a double monopole meander line loaded antenna used for instance in airborne applications in a dome-like configuration.
In each of the above meander line loaded antennas or antenna couplers the meander lines are insulated from a plate or sheet by a dielectric layer. The designs of such antennas or couplers can be maximized for bandwidth by a number of techniques described in the above patents.
Oftentimes, however, VSWR spikes across the broad bandwidth provide the antenna designer with problems when seeking to provide a uniform smooth low VSWR across the entire broadband.
A need, therefore, exists for a broadband meander line antenna assembly which may be small and is broadband with good gain characteristics and yet have a uniform low VSWR across the band.